The present invention is directed to a system and method for controlling the level of signals output from remote antenna units to transmission media in a distributed antenna network. More particularly, the present invention reduces the gain of a remote antenna unit when its output signal level is determined to be greater than the maximum output level which may be received by the transmission media. Accordingly, received signals that are stronger than the maximum output level may be transmitted across the transmission media without saturating the system.
As personal communication services (PCS) evolve as the next generation of cellular telephone technology, systems and techniques for simply and efficiently transmitting and receiving communication signals are being investigated. One known system is a distributed antenna network (which is also referred to as a multicast network) which provides coverage over substantial areas by a plurality of remote antenna units. An example of a distributed antenna network is illustrated in FIG. 1 where an individual transceiver 10 is connected to a plurality of cells 201, . . . 20n by transmission media 30 which transports radio signals between the transceiver unit 10 and the cells 201 . . . 20n. Each of the cells 201, . . . 20n include remote antenna units 211, . . . 21n. The remote antenna units 211, . . . 21n may be connected to the transmission media 30 by frequency converting circuitry 221, . . . 22n for certain applications.
Various infrastructures are being developed and modifications of existing infrastructures are of great interest as alternatives for PCS because they are fully capable of providing high quality signals at lower costs than traditional cellular infrastructures. For example, CATV infrastructures have been modified for use in PCS. Such modifications include the CATV infrastructures using a hybrid fiber/coax (HFC) cable infrastructure to increase capacity and improve service quality. Although it is theoretically possible for any CATV infrastructure to support PCS with the proper modifications, the HFC cable infrastructure offers an attractive option as an economical alternative to wireless providers seeking to avoid the high cost of network construction.
FIG. 2 illustrates the basic components of a CATV infrastructure used to support PCS. In FIG. 2, base station equipments 501 and 502 are connected to a public network such as a public switched telephone network. Remote antenna signal processors (RASPs) 521 and 522 connect the base station equipments 501 and 502 to a fiber equipment 54. The fiber equipment 54 is connected to a fiber node 58 by fiber optic cable 56 and the fiber node 58 is connected to remote antenna driver (RAD) nodes 621 and 622 by two-way coaxial cable 60. The RAD nodes 621 and 622 each include a group of RADs 641 and 642 and 661 and 662 respectively connected to antennas 681, 682, 701, and 702. This CATV infrastructure converts radio frequency signals into CATV frequency signals usable in the existing CATV infrastructure and converts CATV frequency signals back into radio frequency signals for broadcast. More specifically, the RASPs 521 and 522 convert the radio frequency signals from the base station equipments 501 and 502 and then send the converted signals in the downlink path toward the appropriate fiber node 58 and onto the coaxial cable 60.
The RADs 641, 642, 661 and 662 are connected to the coaxial cable 60 for converting CATV frequency signals into assigned radio frequency signals. Radio frequency signals may be received by the RADs 641, 642, 661 and 662 which convert these signals into signals of frequencies suitable for transmission in the uplink path of the CATV infrastructure. Thereafter, the RASPs 521 and 522 convert the upstream CATV frequency signals back into radio frequency signals for processing by the base station equipments 501 and 502. This CATV infrastructure also may accommodate equipment for multiple modulation schemes, such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA).
Radio telephony systems may utilize this CATV infrastructure by operating on available portions of the radio frequency spectrum over fiber optic and coaxial cables which are widely available in urban areas so that such systems may be installed to take advantage of this existing infrastructure. The large installed base of fiber optic and coaxial cables used by CATV operators may thereby be effectively exploited at a minimal cost by this infrastructure which distributes the signals to the appropriate antenna locations. To efficiently provide RF coverage in new cellular and PCS communication systems, it is becoming increasingly common to use distributed antenna systems. In distributed antenna systems, it is often necessary to transmit RF signals covering a wide range of amplitudes across transmission media with a limited and more narrow range of amplitudes for the signals. Conventional automatic gain control (AGC) techniques for controlling the gain are insufficient in allowing the wide range of signals to be fully transmitted over the transmission media of a limited dynamic range. In conventional AGC techniques, even though stronger signals, which exceed the maximum signal level for the transmission media, from sources relatively near the remote antenna units are limited to be below the maximum signal level for transmission across the transmission media, weak signals from sources relatively far away from the remote antenna units are undesirably reduced below the noise floor and effectively cut off. The conventional techniques do not adequately control gain so that strong signals do not saturate the system and weak signals are not unnecessarily reduced below the noise floor.
One object of the present invention therefore is to provide a system and method for controlling the level of signals transmitted from remote antenna units over transmission media in a distributed antenna network so that signals covering a wide range of amplitudes may be transmitted over transmission media having a more narrow range of amplitudes for transmitting signals.
Another object of the present invention is to provide a system and method for comparing a predetermined reference level with the level of signals received by each remote antenna unit in a distributed antenna network, and controlling the gain of the respective remote antenna units based upon this comparison to prevent strong signals from saturating the system.
According to one embodiment of the present invention, the foregoing and other objects are attained in a system and method for controlling the gain of signals transported over transmission media in a distributed antenna network. The system comprises a plurality of remote antenna units where each remote antenna unit includes a signal level comparator for comparing the level of the signals received by the respective antenna unit with a predetermined reference level, and a gain controller for reducing the gain of the remote antenna unit when the signal level comparator determines that the level of the received signal exceeds said predetermined reference level. As a result, the signal levels output from the remote antenna units are prevented from exceeding a maximum signal level so that strong signals do not saturate the system.
In a further embodiment of the present invention, the predetermined reference level is set to correspond with the maximum level for transmitting signals over the transmission media. Thereby, the level of the signals output over the transmission media are prevented from saturating the system without being unduly restricted.
In a still further embodiment of the present invention, the signal level comparator generates an error signal when the received signal exceeds the predetermined reference level which is used for reducing the gain in proportion to the error signal. The system may accommodate TDMA standards when a gain controller is used that is fast enough to respond within the time slots specified by the standards.